1. Field of the Invention
The present invention relates to a laser pulse oscillator for achieving stable oscillation of short pulses with a high repetition rate which are needed for implementing ultrafast optical communication systems, and particularly to a harmonically and regeneratively mode-locked laser pulse oscillator from which is extracted from an output fragment of the laser a clock signal with a frequency of a high-order integer multiple of the fundamental frequency determined by the cavity length of the laser, and which drives at the frequency of the clock signal an optical modulator incorporated in the laser cavity.
2. Description of Related Art
To implement future ultrafast optical communications, a stabilized optical source for generating short pulses with a high repetition rate is expected to play an important role. As a technique for generating picosecond pulse trains in the GHz region, a method of mode-locking a semiconductor laser or that of a fiber laser is known. Although the semiconductor laser is smaller, the fiber laser is superior to the semiconductor laser in the quality of the output pulse (which means transform limited short pulses). However, since the fiber laser has a longer cavity length, it is susceptible to change in temperature, and this makes it difficult to obtain stable operation over a long period of time. This problem has already been solved in part by employing a regenerative mode-locking technique which extracts a clock signal from the output of the laser, and drives a modulator by the clock signal.
FIG. 1 shows a setup of a conventional laser pulse oscillator using a mode-locked optical fiber laser, which is disclosed in laid-open Japanese Patent Application No. 18139/1996. In this figure, the reference numeral 1 designates a rare-earth doped optical fiber, 2 designates a pumping light source for pumping the rare-earth doped optical fiber 1, 3 designates an optical coupler for coupling the pumping light to the rare-earth doped optical fiber 1, 4 designates an optical coupler, 5 designates an optical isolator for confining the traveling direction of the light in a single direction, 6 designates an optical modulator, 7 designates an optical filter, 8 designates an optical coupler, 9 designates a clock extraction circuit, 10 designates a phase shifter, 11 designates an electric amplifier, and 30 designates an optical fiber loop constituting the laser pulse oscillator by linking the elements 1, 3, 4, 5, 6 and 7.
In the laser pulse oscillator as shown in FIG. 1, an optical pulse train with a high repetition rate is generated as follows. Pumping the rare-earth doped optical fiber 1 by the pumping light source 2 through the optical coupler 3 causes continuous wave light to be oscillated in the forward direction of the optical isolator 5 within the passband of the optical filter 7. The laser output is extracted through the optical coupler 4 and split by the optical coupler 8 so that a fragment of the laser output is input to the clock extraction circuit 9 which includes a photo detector, a narrow-band electric filter and an electric amplifier. The clock extraction circuit 9 extracts from the fragment of the laser output a sinusoidal clock signal with a particular frequency corresponding to a high-order integer multiple of a fundamental frequency determined by the cavity length of the laser. The clock signal undergoes phase adjustment through the phase shifter 10, is amplified by the electric amplifier 11, and applied to the optical modulator 6. Thus, intensity modulation of the light is carried out in the cavity at a frequency locked to the clock signal.
Generally speaking, the mode-locking in the fundamental cavity mode can be achieved by amplitude modulation at a fundamental frequency of f.sub.0 =c/(nL) determined by the cavity length, where L is the cavity length, n is the refractive index of the optical fiber, and c is the traveling speed of the light. Besides, the harmonic mode-locking at a frequency of q times the fundamental cavity mode can be obtained by setting the modulation frequency f at f=qf.sub.0 =qc/(nL), q times the fundamental frequency f.sub.0 determined by the cavity length L of the laser, where q is an integer. In other words, q optical pulses are generated at equal intervals in the cavity of the laser, producing an optical pulse train at the repetition-rate frequency of qf.sub.0.
Let us consider a case of employing a clock extraction circuit which operates at 10 GHz as the clock extraction circuit 9. Although the clock extraction process extinguishes clock signals with a frequency around 10 GHz, which disagrees with integer multiples of the fundamental frequency f.sub.0 because they cannot sustain a stable optical pulse train, it gradually reinforces a stable pulse oscillation by the clock signal with a frequency coincident with the integer multiple of the fundamental frequency f.sub.0 because the modulation frequency f completely coincides with the repetition rate of the optical pulse train. In the course of repeating this process, only a particular clock signal remains around 10 GHz which is initially noisy but whose frequency coincides with the integer multiple of the fundamental frequency. In other words, only one clock signal grows to drive the optical modulator 6, thus achieving harmonic mode-locking at 10 GHz. The conventional technique is called harmonic and regenerative mode-locking.
In the conventional harmonically and regeneratively mode-locked laser pulse oscillator, even if the repetition rate fluctuates because of the cavity length variations due to a temperature change or the like, the repetition rate of the optical pulse train does not deviate from the modulation frequency. This is because the modulation is always carried out by the clock signal locked by itself to the repetition rate of the output optical pulse train. This enables a stable optical pulse train with a high repetition rate to be generated over a long time period without inducing degradation of the waveform of the optical pulses due to a temperature change or the like.
The conventional laser pulse oscillator as shown in FIG. 1, however, has a problem to be solved in that although it can generate a stable optical pulse train over a long period of time, the repetition rate of the optical pulses slightly varies owing to fluctuation in the length of the cavity, and hence the repetition-rate frequency cannot be kept at a fixed value. In addition, there is another problem to be solved in that the repetition-rate frequency of the laser pulse oscillator cannot be locked to an external signal because the laser pulse oscillator oscillates by itself.
The problems will be explained in more detail. In the conventional harmonically and regeneratively mode-locked laser pulse oscillator, the cavity length varies owing to change in the temperature or change in the laser cavity, resulting in fluctuations of the fundamental cavity mode. The modulation frequency driving the optical modulator automatically follows variation in the cavity length thereby enabling a stable pulse oscillation over a long period of time. The repetition-rate frequency of the optical pulses, however, slightly varies in such a manner that the repetition-rate frequency reduces with an increase in the cavity length, and increases with an increase in cavity length. For example, when L=200 m and f=10 GHz, the cavity length varies by 20 .mu.m for each variation of 0.01.degree. C. in the cavity temperature, resulting in 1 kHz variation in the repetition rate frequency.
FIG. 2 shows an example of frequency fluctuations in the conventional laser pulse oscillator with time. As seen from FIG. 2, the frequency fluctuates at random with time in accordance with a variation in cavity length. In addition, since the laser pulse oscillator oscillates in a self-sustained mode at the modulation frequency determined by a variation in cavity length in this state, the repetition-rate frequency cannot be locked to the external signal.
As described above, although the conventional technique can generate a stable optical pulse train with a high repetition rate, the repetition rate frequency of the optical pulses fluctuates, and it is difficult to generate an optical pulse train locked to the external signal at a fixed frequency.
To solve the problem in that the repetition rate frequency fluctuates with time because the laser pulse oscillator as shown in FIG. 1 oscillates in a self-sustained mode, a laser pulse oscillator applying an offset locking method is disclosed in Japanese laid-open Patent Application No. 139536/1997 or A Repetition-Rate Stabilized and Tunable, Regeneratively Mode-Locked Fiber Laser Using an Offset-Locking Technique, Jpn. J. Appl. Phys. Vol. 35 (1996) pp. L691-L694 Part 2, 6A, Jun. 1, 1996. This method detects the frequency difference between an external signal and the laser clock signal, and converts the frequency difference into a voltage used as a reference voltage for applying offset.
FIG. 3 shows the setup of the laser pulse oscillator. In FIG. 3, the laser pulse oscillator comprises the rare-earth doped optical fiber 1, the pumping light source 2 for pumping the rare-earth doped optical fiber 1, the optical coupler 3 for coupling the pumping light to the rare-earth doped optical fiber 1, the optical coupler 4 for extracting the output, the optical isolator 5 for confining the traveling direction of the light to one direction, the optical modulator 6, the optical filter 7, the optical coupler 8, the clock extraction circuit 9, the phase shifter 10, the electric amplifier 11, an electric amplifier 16, an external signal generator (synthesizer) 12, a frequency difference detector 13, a frequency-to-voltage converter 14, a differential amplifier 15, a piezoelectric transducer (PZT fiber stretcher) 17, and the optical fiber loop 30 linking the elements 1, 3, 4, 5, 6, 7 and 17 to constitute the laser cavity. Pumping the rare-earth doped optical fiber 1 by the pumping light source 2 through the optical coupler 3 causes continuous wave light to be oscillated in the forward direction of the optical isolator 5 within the transmission band of the optical filter 7. The laser output is extracted through the optical coupler 4 and split by the optical coupler 8 so that a fragment of the laser output is input to the clock extraction circuit 9 including a photo detector, a narrow-band electric filter and an electric amplifier. The clock extraction circuit 9 extracts from the fragment of the laser output a sinusoidal clock signal with a particular frequency. The clock signal undergoes phase adjustment through the phase shifter 10, is amplified by the electric amplifier 11, and applied to the optical modulator 6. As a result, the regenerative mode-locking is achieved, generating an optical pulse train. Thus, the intensity modulation of the light is carried out in the cavity at a frequency locked to the clock signal. Since the modulation signal is derived from the light emitted from the laser, the light is always optimally modulated, and a stable pulse oscillation is sustained over a long time.
The stabilization of the repetition rate frequency in the laser pulse oscillator as shown in FIG. 3 will be described. Receiving the clock signal from the clock extraction circuit 9 and the output of the synthesizer 12 with a fixed frequency, the frequency difference detector 13 detects the frequency difference between the two input signals. Then, the frequency-to-voltage converter 14 generates a voltage V corresponding to the frequency difference .DELTA.f=.vertline.f.sub.1 -f.sub.2 .vertline., where f.sub.1 is the frequency of the clock signal and f.sub.2 is the frequency of the external signal, so that the offset is applied using the voltage V. More specifically, as the laser repetition rate frequency f.sub.1 reduces, .DELTA.f reduces and the voltage V drops, whereas as f.sub.1 increases, .DELTA.f increases and the voltage V rises. The differential amplifier 15, receiving the voltage V(t) corresponding to .vertline.f.sub.1 -f.sub.2 .vertline. at its first input and a reference dc voltage V.sub.ref at its second input, amplifies the difference .DELTA.V(t)(=V(t)-V.sub.ref) between the two inputs. Estimating the relationship between the extension and contraction of the piezoelectric transducer (PZT fiber stretcher) 17 and the applied voltage in advance enables the sign of a control voltage signal to be adjusted so that f.sub.1 is reduced when it increases and .DELTA.V(t) becomes positive. The amplified .DELTA.V(t) is applied to the piezoelectric transducer 17 through the electric amplifier 16 to perform negative feedback for automatic control. When f.sub.1 reduces, the negative feedback is automatically applied to increase f.sub.1. Thus, the feedback circuit sets the laser repetition rate frequency at the frequency shifted by an amount of .DELTA.f from that of the external signal.
According to the laser pulse oscillator as shown in FIG. 3, the laser repetition rate frequency can be set at the frequency shifted by the predetermined frequency difference from that of the external signal. The offset locking method, however, cannot equalize the laser repetition rate frequency to that of the external signal. As a result, it cannot generate an optical pulse train locked to the external signal.